The present invention relates to channel allocations in RF communications systems and specifically to a robust channel allocation method for PTMP (point-to-multi-point) systems.
In the specification and claims which follow below, the terms “master” and “node” are intended to mean both a schematic/configuration position within an RF network and one or more RF devices typically transmitting and/or receiving at or associated with the respective schematic/configuration position. Typically, the “device” is a radio device. Typically in a PTMP system, the master is the central, coordinating point whereas one or more nodes comprise “multi points”.
Reference is now made to FIG. 1, which is schematic time line diagram of signals 10 in a prior art master-node communications system. In low power wireless communications systems with star topology—such as a PTMP system—where many nodes need to stay connected to a master, one of the options is to have the master transmit a beacon 12 (also called a “sync beacon”) which is “visible” to all of the nodes, as known in the art. The purpose of the sync beacon is to serve to allow high precision clock synchronization for individual node transmission activities, thereby ensuring orderly overall communication system functioning. In the time between consecutive beacon 12 transmission, a period 14 is identified in which RF activity takes place, such as transmissions/receptions by individual nodes and/or the master. Depending on the system size and characteristics, the beacon is typically transmitted at a frequency on the order of approximately many times a second, although the frequency may be higher or lower. When transmission by a node is necessary, the node “searches” (i.e. waits to receive) for the sync beacon. Referring to the figure, once beacon 12 is found, the node can be synchronized with relatively high time resolution, as known in the art, to transmit at/in an allocated, predefined slot 18.
In the specification and claims which follow below, the terms “slot” and “channel” are intended to mean the typical time allocation, as know in the art, for respective, individual node activity (i.e. transmission and/or reception) within the network. Nodes may use slotted ALOHA, slotted CSMA or any other mechanism/algorithm as know in the art to resolve potential transmission “collisions”, i.e. when two or more nodes undesirably substantially simultaneously transmit on the same slot.
Depending on the system size and characteristics there can be on the order of approximately 20 to 500 slots allocated between beacons, with a time duration of the slot ranging typically from approximately 200 to 2,000 microseconds. A sync beacon is useful, inter alia, in systems employing nodes with reduced or limited power consumption, as described hereinbelow.
US Patent Application no. 20080232286 by Habetha et al., whose disclosure is incorporated herein by reference, describes a system and method for hibernation mode for beaconing device. Habetha describes, inter alia, a way to inform the network on a beacon allocation of other device that is currently in a low power mode.
US Patent Application 20080095126 by Mahany et al., whose disclosure is incorporated herein by reference, describes a low-power wireless beaconing network supporting proximal formation, separation and reformation. US Patent Application no. 20080165761, by Goppner et al., whose disclosure is incorporated herein by reference, describes a method for synchronization and data transmission in a multi-hop network. These prior art all describe and/or relate to beaconing.
A standard called “ZigBee”, whose disclosure is incorporated herein by reference, is also related to low power wireless communications systems using a beacon. ZigBee, developed by the ZigBee Alliance, is a very low-cost, very low-power consumption, two-way, wireless communications standard. Applications adopting the ZigBee standard are to be embedded in consumer electronics, home and building automation, industrial controls, PC peripherals, medical sensor applications, toys, and games. At its core, Zigbee (http://www.zigbee.org/en/) describes a mesh network architecture. Wireless mesh networks were originally developed for military applications but have undergone significant evolution in the past decade. As the cost of radios has plummeted, single radio products have evolved to support more radios-per-mesh node. Additional radios per mesh node additional provide specific functions, such as client access, backhaul service, or scanning radios for high speed handover in mobility applications. Mesh node design also became more modular, meaning one box could support multiple radio cards, each card operating at a different frequency.
When a node is operated in a very low power consumption mode, meaning it is normally powered off for long periods of time and/or powered on for very short periods of time; effective node synchronization within the network becomes more difficult. A problem exists when one or more nodes in a network cannot find/receive the sync beacon for conventional synchronization and subsequent slotted transmission activity. In the specification and claims which follow, the term “resolve”, as used in “resolve the beacon”, is intended to mean that the node has received the sync beacon and has been synchronized with the master in relatively high time resolution, as known in the art, to transmit at/in an allocated slot.
There is therefore a need for a way to allocate channels for one or more nodes operating in a low power consumption mode, taking advantage of available RF spectrum, even when the sync beacon cannot be resolved by a node.